Plastics Technology

DEC 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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EXTRUSION Screw and barrel wear can be one of the most expensive oper- ating costs in an extrusion process. But it can be reduced or even eliminated if you understand the cause. Wear of this type is caused by several things. Assuming the screw and barrel materials are compatible—there are many fully developed and tested mate- rials available—let's focus on other possible causes of screw wear, and then talk about how you can distinguish one type from the other. In previous columns I have noted that one of the most serious wear effects is caused by wedging, which is an unbalanced radial pressure in the screw that aggressively forces the screw against the opposite barrel wall. Wedging occurs when the melting rate is exceeded to such an extent that the screw essentially gets momentarily plugged with solid polymer. A high pressure develops on one side of the barrel, with much lower pressure on the other side. This is largely a screw-design issue and generally cannot be corrected by changing operating conditions, such as simply increasing the barrel temperatures. Slowing the rpm is about the only effective measure, and that is always undesirable due to the lost output. A second cause of wear is abrasive fillers that get caught between the flight O.D. and the barrel wall. These particles—even though they are considered much softer than the flight and barrel material—can erode the screw flight and barrel, even at relatively low pressures. It's like sand washing over the rocks on a beach. The third type of wear is related to barrel alignment. When the screw has to rotate in a cylindrical space that is not straight, it has to bend with each revolution. The bending force is enormous and again aggressively forces the barrel and screw flights together under great pressure, causing a crushing of the Telltale Signs of Screw Wear Determining the cause of wear is the first step in eliminating it. By Jim Frankland What's Causing Your Screw Wear? The causes of wear can be determined by examining worn flights. If you have a burr on the trailing side of the flight (top), wedging is most likely the cause. Wear caused by abrasion will reveal itself by a polishing or rounding of the flight corners on the pushing side of the flight surface (bottom), eroding the base screw material. Misalignment results in a tearing and a crushing action that produces a burr on both sides of the flight (center). I have found over the years that there are telltale signs of the cause of wear if you examine the worn screw flights. Flow → Flow → Flow → 30 DECEMBER 2018 Plastics Technology K now How

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